A resin-encapsulated electronic product is generally manufactured through such processes as described below. Specifically, as shown in FIG. 2A, the product is assembled such that a double-side printed circuit substrate 1, on both sides of which predetermined printed patterns are formed, is mounted with necessary electronic components 2-4 by soldering and is connected with lead elements 7 at its both ends, so as to have certain required functions, for example, as a gas meter or other appliances. Next, as shown in FIG. 2B, the entire printed circuit substrate 1 including the electronic components 2-4 is inserted into a cavity 10 formed by a lower mold 8 and an upper mold 9, after which both molds 8, 9 are clamped together. Successively, a thermoplastic resin material 12 which has been heated at a high temperature and thus melted is injected into the cavity 10 through an injection orifice 11. After the thermoplastic resin 12 has been set by being cooled, the molds 8, 9 are removed, resulting in a resin-encapsulated electronic product 13 which is covered with a package layer 17 of the thermoplastic resin 12 and thereby protected from an outer atmosphere as shown in FIG. 2C.
However, the above-described conventional manufacturing method has such a drawback that when the thermoplastic resin 12 is injected into the cavity 10 from the orifice 11, the heated and melted resin 12 tends to cause various defects: for example the electronic components 2-4 may be damaged by heat; a part of solder paste which connects the electronic components 2-4 on the substrate 1 may melt and flow away; or electronic components may be shoved and thus displaced from a certain mounting position. There has been a problem of decrease in the yield of such resin-encapsulated electronic products 13 due to these defects.